The present invention relates to an optical element for a gas laser and also to a gas laser apparatus using the same. More particularly, the present invention relates to an optical element for an ultraviolet gas laser to be used in a semiconductor exposure apparatus having an excimer laser or a molecular fluorine laser and also to a gas laser apparatus using the same.
(Exposure Light Source)
In the trend of making semiconductor integrated circuits finer and more integrated, an improved resolution is required for semiconductor exposure apparatus. To meet this demand, efforts are being paid to use shorter wavelength for the light emitted from an exposure light source. A gas laser apparatus is being popularly employed as exposure light sources in place of conventional mercury lamps. KrF excimer laser apparatus for emitting deep ultraviolet rays of a wavelength of 248 nm and ArF excimer layer apparatus for emitting vacuum ultraviolet rays of a wavelength of 193 nm are being used as gas laser apparatus for exposure. Attempts are being made to apply a liquid immersion technique of reducing the apparent wavelength of an exposure light source by filling the gap between an exposure lens and a wafer, thereby shifting the refractive index, to an ArF excimer laser apparatus as an exposure technique of the next generation. With ArF excimer laser liquid immersion, the wavelength is reduced to 134 nm when immersed in pure water. F2 laser liquid immersion exposure may possibly be adopted for F2 (molecular fluorine) laser apparatus that emit vacuum ultraviolet rays of a wavelength of 157 nm as exposure light sources of the third generation. The wavelength is believed to be made equal to 115 nm by F2 laser liquid immersion exposure.
(Exposure Optical Element and Chromatic Aberration)
Many semiconductor exposure apparatus adopt a projection optical system as the optical system thereof. In a projection optical system, optical elements such as lenses having different refractive indexes are combined to correct the chromatic aberration. At present, optical materials that are suitable as lens materials of projection optical systems for the wavelength (ultraviolet) range between 248 nm and 157 nm of lasers operating as exposure light sources are only synthetic quartz and CaF2. For this reason, monochromatic lenses of the total refraction type that are formed only by synthetic quartz are adopted as projection lenses for KrF excimer lasers, whereas partially achromatic lenses of the total refraction type that are formed by synthetic quartz and CaF2 are adopted as projection lenses for ArF excimer lasers. However, the natural oscillation spectrum line width of both KrF excimer lasers and ArF excimer lasers is as wide as about 350 to 400 pm so that, when such a projection lens is used, chromatic aberration occurs to reduce the resolving power. Therefore, the spectrum line width of the laser beams emitted from such gas laser apparatus needs to be narrowed to such a degree at which the chromatic aberration can be disregarded. For this reason, a band narrowing module having a band narrowing element (etalon, grating or the like) is arranged in the laser resonators of such gas laser apparatus to realize band narrowing of the spectrum line width.
(Liquid Immersion Lithography and Polarized Light Illumination)
As described above, in the case of ArF excimer laser liquid immersion lithography, the refractive index will be 1.44 when H2O is employed as medium so that the lens numerical aperture NA that is proportional to the refractive index can be theoretically increased to 1.44 times of the conventional numerical aperture. As the NA is increased, the influence of the degree of polarization of the laser beam that is the light source will increase. While there is no influence in the case of TE polarized light whose direction of polarization is parallel to the direction of the mask pattern, the image contrast will become low in the case of TM polarized light whose direction of polarization is orthogonal to the direction of the mask pattern. This is because the direction of the electric field vector at the focal point on the wafer is different in the latter case so that the intensity becomes weak as the incident angle to the wafer increases if compared with the former case where the direction of the electric field vector is same and identical. The influence thereof is intensified when the NA approaches or exceeds 1.0 and ArF excimer laser liquid immersion falls into such a case. Therefore, a desired state of polarization needs to be controlled for the illumination system of an exposure apparatus as described above. To control such polarized light illumination, the polarization of the laser beam input to the illumination system of the exposure apparatus is required to be in a linearly polarized state. The degree of polarization is the ratio of the linear polarization and the non-linear polarization that are measured and the polarization of a laser beam is required to maintain a high degree of polarization.
(Prior Art for Raising Polarization Purity)
The techniques described in Patent Document 1 and Patent Document 2 are known as techniques for raising the degree of polarization of a laser beam.
The technique described in Patent Document 1 provides a method of preventing degree of polarization from being degraded by birefringence that arises when light passes the inside of an optical element by making a laser beam to be transmitted perpendicularly to the cleavage crystal face (111) of calcium fluoride crystal of the optical element such as a beam expander prism or a front mirror to be used for a laser.
The technique described in Patent Document 2 provides a method of preventing degree of polarization from being degraded by intrinsic birefringence that arises when light passes the inside of an optical element by making the optical axis of a laser beam to be transmitted perpendicularly relative to the (100) crystal face of the calcium fluoride crystal of the optical element used for a laser.
However, the above-described conventional art has the following problem.
Patent Document 1 does not describe any specific means for actually making an optical axis pass perpendicularly relative to the (111) crystal face of a window that operates as optical element and also making the surface thereof show the Brewster angle and then a crystal orientation face of calcium fluoride crystal that is hard to a certain extent cannot be selected as the face to be cut to produce a window if the above two objectives are to be achieved simultaneously so that it is not possible to realize high precision polishing with low surface coarseness. If it is not possible to polish with low surface coarseness, flaws referred to as latent flaws are left in a region within sub-microns from the polished surface. Such latent flaws absorb the laser beam irradiated from a laser to damage the crystal surfaces and generate a defect of allowing fluorine to come out to consequently give rise to a problem that the crystal can no longer be used as window for a laser chamber.
The technique described in Patent Document 2 is to prevent degradation of degree of polarization due to intrinsic birefringence by arranging a laser beam to pass perpendicularly relative to the (100) crystal face of an optical element. Stress birefringence that arises when stress is applied is largest in the direction that is perpendicular relative to the (100) crystal face and, when it is used as chamber window, stress birefringence can possibly take place due to the stress that arises when holding the window, the gas pressure of several atmospheric pressures in the chamber and/or the stress caused by the thermal stress that arises by laser beam irradiation. Additionally, a cut surface is produced at an angle of 17.58° or 26.76° relative to the (111) crystal face and cut surfaces are used as the opposite surfaces of the chamber window so that the following two problems arise. One is that, since the surface coarseness of the cut surfaces does not allow small high precision polishing to reduce the threshold value for the surface damage caused by laser beam irradiation. The other is that, when used as laser chamber window, it is subjected to gas pressure of about 4,000 hPa so that it can be broken at the (111) crystal face that is apt to be cleaved. Furthermore, when the cut surface is produced at 17.58° relative to the (111) crystal face, the angle formed by the chamber window and the optical axis is 70° and hence Fresnel reflection of P-polarized light and that of S-polarized light are 4.2% and 30.0% respectively so that, although the P-polarized light component is selected, the Fresnel reflection of P-polarized light is large as a result of being transmitted through the window to make it impossible to secure the laser output.
Thus, Patent Document 2 discloses a technique of preventing degradation of degree of polarization due to intrinsic birefringence and stress birefringence from taking place by means of an optical element for an ultraviolet gas laser such as a window made of calcium fluoride crystal and having two faces, one of which, or face 2, is adapted to receive ultraviolet rays entering through it and exiting from the other face and at least one of which is parallel to the (110) crystal face of the calcium fluoride crystal and also preventing cracks and defects from arising by laser beam irradiation by smoothing the cut surfaces.    [Patent Document 1] JP-A-11-177173    [Patent Document 2] U.S. Patent Application Publication No. 2003/219056 Specification    [Patent Document 3] JP-A-2002-353545    [Patent Document 4] JP-A-2006-073921
Thus, the technique disclosed in Patent Document 4 is designed to prevent degradation of degree of polarization due to intrinsic birefringence and stress birefringence and also prevent to a certain extent the surface damage of calcium fluoride crystal that can be caused by laser beam irradiation by means of high precision polishing of reducing the surface coarseness to a certain extent by selecting the (110) crystal face of the calcium fluoride crystal as the face to be cut.
In view of the above-identified problems of the prior art, it is therefore the object of the present invention to reduce latent flaws and prevent the surface damage of calcium fluoride crystal that can be caused by laser beam irradiation by cutting the calcium fluoride crystal along the crystal face of the (111) crystal face that is hardest and subjecting the crystal face to high precision polishing for low surface coarseness. Then, the ultimate object of the present invention is to provide an optical element for a gas laser such as Brewster window or beam expander prism formed by using calcium fluoride crystal that can reduce the Fresnel reflectance of P-polarized light, realize a high degree of polarization and also suppress degradation of itself due to intense ultraviolet (ArF in particular) laser beam irradiation and also a gas laser apparatus.